Bone is a dynamic biological tissue composed of metabolically active cells that are integrated into a rigid framework. The healing potential of bone, whether in a fracture or fusion model, is influenced by a variety of biochemical, biomechanical, cellular, hormonal, and pathological mechanisms. A continuously occurring state of bone deposition, resorption, and remodeling facilitates the healing process. The success of many spine operations depends on the restoration of long-term spinal stability. Whereas spinal instrumentation devices may provide temporary support, a solid osseous union is desired to provide long term stability. The failure of fusion to occur may result in the fatigue and failure of supporting instrumentation and persistence or worsening of symptoms.
Bone metabolism is regulated by a host of hormonal and local factors. Three of the calcitropic hormones that affect bone metabolism are parathyroid hormone, vitamin D, and calcitonin. Bone metabolism is also affected by a series of proteins, or growth factors, released from platelets, macrophages, and fibroblasts. These proteins cause healing bone to vascularize, solidify, incorporate, and function mechanically. They can induce mesenchymal-derived cells, such as monocytes and fibroblasts, to migrate, proliferate, and differentiate into bone cells. The proteins that enhance bone healing include the BMPs, insulin-like growth factors, transforming growth factors, platelet derived growth factor, and fibroblast growth factor, among others.